WO2016121371A1 - Case hardened steel - Google Patents

Case hardened steel Download PDF

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Publication number
WO2016121371A1
WO2016121371A1 PCT/JP2016/000359 JP2016000359W WO2016121371A1 WO 2016121371 A1 WO2016121371 A1 WO 2016121371A1 JP 2016000359 W JP2016000359 W JP 2016000359W WO 2016121371 A1 WO2016121371 A1 WO 2016121371A1
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Prior art keywords
fatigue strength
steel
content
range
gear
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PCT/JP2016/000359
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French (fr)
Japanese (ja)
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WO2016121371A8 (en
Inventor
佳祐 安藤
福岡　和明
冨田　邦和
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Ｊｆｅスチール株式会社
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Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to JP2016528912A priority Critical patent/JP6226071B2/en
Priority to US15/546,098 priority patent/US11702716B2/en
Priority to EP16742975.2A priority patent/EP3252182B1/en
Priority to MX2017009674A priority patent/MX2017009674A/en
Priority to KR1020177023524A priority patent/KR101984041B1/en
Priority to CN201680005469.1A priority patent/CN107532252B/en
Publication of WO2016121371A1 publication Critical patent/WO2016121371A1/en
Publication of WO2016121371A8 publication Critical patent/WO2016121371A8/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/28—Normalising
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces

Definitions

  • the present invention relates to a case-hardened steel used by carburizing and quenching, and in particular, a boron-containing case-hardened steel excellent in fatigue resistance and impact resistance, which can be applied to drive transmission parts such as automobiles.
  • surface hardening heat treatments such as carburizing, nitriding, and carbonitriding are performed on parts that require high fatigue strength and wear resistance in machine parts used as automobiles, construction machines, and other various industrial machines.
  • case-hardened steel such as SCr, SCM, SNCM, etc. is usually used in JIS standards, and after forming into the desired part shape by machining such as forging or cutting, the above-mentioned surface hardening heat treatment is applied. Then, it is manufactured into parts through a finishing process such as polishing.
  • Patent Document 1 discloses a case-hardened boron steel that can suppress the coarsening of crystal grains with TiN while adding Ti to fix N in the form of TiN and securing solid solution B. .
  • Patent Document 2 proposes to improve toughness by adjusting the addition amount of Si, Mn and Cr in the Ti-added boron steel and reducing the carburizing abnormal layer depth.
  • Patent Document 3 discloses a method for producing a case-hardened boron steel that suppresses the formation of BN by adding a large amount of Al and prevents abnormal grain growth of fine grains by fine carbonitride obtained by heat treatment before carburizing. It is disclosed.
  • Patent Document 4 discloses a case hardening steel excellent in cold forgeability that suppresses the occurrence of an abnormal carburizing layer by addition of Sb and effectively suppresses coarsening of crystal grains by Ti-Mo carbide. Is disclosed. Further, Patent Document 5 discloses a steel for machine structural use that suppresses the thickness of the decarburized layer by adding Sb and has a cold workability equivalent to that of a steel material that has been subjected to conventional soft annealing, and a method for manufacturing the same. .
  • JP-A-57-070261 JP 58-120719 A Japanese Patent Laid-Open No. 2003-342635 JP 2012-62536 A Japanese Patent Laid-Open No. 2004-250767
  • each of the inventions described in Patent Documents 1 to 4 has the following problems.
  • N is fixed in the form of TiN, and it is considered that B does not combine with N.
  • TiN exists in steel as a relatively large square inclusion, this becomes a starting point of fatigue, and in gears, surface fatigue such as pitching and bending fatigue strength at the root are reduced.
  • the square TiN reduces the impact resistance of the gear and may cause breakage of the gear when an impact load is applied to the gear.
  • Patent Document 5 depending on the balance between Sb having a decarburization-inhibiting effect and Si that promotes decarburization, it is difficult to reliably avoid the reduction of carbon on the surface layer, and desired characteristics can be obtained. There is a problem that can not be.
  • an object of the present invention is to solve the above-described problems and to provide a case-hardened steel having excellent fatigue characteristics at a relatively low production cost.
  • the present invention is based on the above findings. That is, the gist configuration of the present invention is as follows. 1. % By mass C: 0.10 to 0.30%, Si: 0.10 to 1.20% Mn: 0.30-1.50% S: 0.010 to 0.030%, Cr: 0.10 to 1.00%, B: 0.0005-0.0050%, Sb: 0.005 to 0.020% and N: 0.0150% or less are included within a range satisfying the following formula, When B- (10.8 / 14) N ⁇ 0.0003%, 0.010% ⁇ Al ⁇ 0.120% and B- (10.8 / 14) N ⁇ 0.0003%, 27/14 [((N ⁇ (14 / 10.8 ) B + 0.030] ⁇ Al ⁇ 0.120%, the balance is iron and inevitable impurities, Ti in the inevitable impurities, Ti: Case-hardened steel characterized by being 0.005% or less. Sb ⁇ ⁇ Si / 2 + (Mn + Cr) / 5 ⁇ / 70
  • C 0.10 to 0.30%
  • C 0.10 to 0.30%
  • the toughness of the core part decreases. Therefore, the C content is limited to the range of 0.10 to 0.30%. Preferably it is 0.15 to 0.25% of range.
  • Si 0.10 to 1.20%
  • Si is an element effective for increasing the softening resistance in a temperature range of 200 to 300 ° C., which is estimated to be reached during rolling of gears and the like. It also has an effect of suppressing the formation of coarse carbides during carburization, and at least 0.10% addition is essential.
  • Si is a ferrite stabilizing element, and excessive addition raises the Ac 3 transformation point, and in the normal quenching temperature range, ferrite tends to appear in the core portion having a low carbon content, and at the tooth base. Since the bending fatigue strength decreases, the upper limit was made 1.20%. Preferably it is 0.20 to 0.60% of range.
  • Mn 0.30 to 1.50%
  • Mn is an element effective for improving the hardenability and needs to be added at least 0.30%.
  • Mn tends to form an abnormal carburization layer, and excessive addition causes an excessive amount of retained austenite and leads to a decrease in hardness, so the upper limit was made 1.50%.
  • the upper limit was made 1.50%.
  • it is 0.50 to 1.20% of range.
  • S 0.010-0.030% S has a function of forming sulfides with Mn and improving machinability, so it is contained at least 0.010% or more. On the other hand, excessive addition reduces the fatigue strength and toughness of the parts, so the upper limit was made 0.030%.
  • Cr 0.10 to 1.00% Cr is an element effective for improving not only hardenability but also temper softening resistance, and if the content is less than 0.10%, its addition effect is poor. On the other hand, when it exceeds 1.00%, it becomes easy to form a carburized abnormal layer. Further, the hardenability becomes too high, the toughness inside the gear is deteriorated, and the bending fatigue strength is lowered. Therefore, the Cr content is limited to the range of 0.10 to 1.00%. Preferably it is 0.10 to 0.60% of range.
  • B 0.0005-0.0050%
  • B is an element effective for ensuring hardenability by adding a small amount, and needs to be added at least 0.0005%.
  • the amount of B is limited to the range of 0.0005 to 0.0050%. Preferably it is 0.0010 to 0.0040% of range.
  • Sb 0.005-0.020% Since Sb has a strong tendency to segregate at grain boundaries, Sb is an important element for suppressing surface reaction such as deboronization and nitriding (BN formation) during carburizing treatment and ensuring hardenability. In order to obtain the effect, the addition of at least 0.005% is essential. However, excessive addition not only leads to an increase in cost but also reduces toughness, so the upper limit was made 0.020%. Preferably it is 0.005 to 0.015% of range.
  • the hardness is lowered with a decrease in the hardenability in the peripheral portion, and fatigue failure starting from the hardness tends to occur.
  • the lower limit of the amount of Sb having the effect of suppressing grain boundary oxidation as indicated by the right side of the above formula according to the content of Si, Mn, Cr, ensuring hardenability in the surface layer It is possible to suppress a decrease in fatigue strength.
  • N 0.0150% or less N is an element that combines with Al to form AlN and contributes to the refinement of austenite crystal grains. Therefore, it is preferable to add at 0.0030% or more. However, when added in excess, not only is it difficult to secure the solid solution B, but also bubbles are generated in the steel ingot during solidification and deterioration of forgeability is caused, so the upper limit is made 0.0150%.
  • the content of Al is specified as follows according to the amount of B.
  • B- (10.8 / 14) N ⁇ 0.0003%: 0.010% ⁇ Al ⁇ 0.120%
  • Al is an element necessary as a deoxidizing agent, and at the same time, in the present invention, it is necessary to secure solid solution B.
  • solid solution B amount represents the remaining B amount after subtracting the stoichiometric amount of B that binds to N from the contained B (hereinafter also referred to as solid solution B amount). Yes. If this solid solution B amount is 0.0003% or more, it becomes possible to secure the solid solution B necessary for improving the hardenability.
  • the Al content is set to a range of 0.010% or more and 0.120% or less.
  • the balance of the above-described components is iron and inevitable impurities, and Ti among these impurities must be suppressed according to the upper limit shown below.
  • Ti 0.005% or less Ti has a strong bonding force with N and forms TiN. However, since TiN exists in steel as a relatively large square inclusion, this becomes a starting point of fatigue, and in gears, surface fatigue such as pitching and bending fatigue strength at the root are reduced. Accordingly, in the present invention, Ti is an impurity, and it is preferable that Ti be as small as possible. Specifically, if it exceeds 0.005%, the above-mentioned adverse effects appear, so the Ti amount is limited to 0.005% or less.
  • P and O are mentioned as inevitable impurities. That is, P is segregated at the grain boundary and causes the carburized layer and the internal toughness to be lowered. Specifically, when the content exceeds 0.020%, the above-described adverse effects appear, so the P content is preferably set to 0.020% or less.
  • O is an element that exists as an oxide inclusion in steel and impairs fatigue strength. Like TiN inclusions, the lower the content, the lower the fatigue strength and toughness. Specifically, if it exceeds 0.0020%, the above-described adverse effects appear, so the O content is preferably 0.0020% or less.
  • Nb 0.050% or less Nb may be added to refine crystal grains and strengthen grain boundaries to contribute to improving fatigue strength.
  • Nb is added, it is preferably contained at least 0.010% or more.
  • the effect is saturated at 0.050%, and addition of a large amount increases the cost, so the upper limit is preferably made 0.050%.
  • V 0.200% or less
  • V is an element that improves hardenability and increases temper softening resistance like Si and Cr, and also has an effect of forming carbonitrides and suppressing coarsening of crystal grains.
  • it is preferable to add at 0.030% or more. Further, the effect is saturated at 0.200%, and the addition of a large amount increases the cost. Therefore, when added, the content is preferably 0.200% or less.
  • a free cutting element such as Pb, Se, or Ca may be included as necessary.
  • the suitable manufacturing conditions are as follows.
  • a steel material having the above-described component composition is melt-cast to form a billet, and this billet is hot-rolled and then preformed to form a gear.
  • it is machined or machined after forging to form a gear shape, and then carburized and quenched, and if necessary, the tooth surface is further polished to obtain a final product.
  • shot peening or the like may be added.
  • the carburizing and quenching treatment is preferably performed at a carburizing temperature of 900 to 1050 ° C., a quenching temperature of 800 to 900 ° C., and tempering within a range of 120 to 250 ° C.
  • Table 2 shows the survey results for each of the survey items described above.
  • the steels of the present invention (Nos. 1 to 15) have the same or better properties than SCr420 (No. 34) in both rotational bending and gear fatigue characteristics, and are superior to the comparative steels (No. 16 to 33). I understand.
  • Comparative Steel No. 16 since the comparative steel No. 16 had a C content lower than the range of the present invention, the internal hardness was too low, and the rotational bending fatigue strength and the gear fatigue strength were reduced. Since the comparative steel No. 17 had a C content higher than the range of the present invention, the toughness of the core portion was reduced, and the rotary bending fatigue strength and the gear fatigue strength were reduced. In Comparative Steel No. 18, since the Si content was lower than the range of the present invention, the resistance to tempering softening decreased and the gear fatigue strength decreased. Comparative Steel No. 19 has a Si content lower than the range of the present invention and a Cr content higher than the range of the present invention.
  • Comparative steel No. 20 has a Si content higher than the range of the present invention. Therefore, ferrite was generated inside, bending fatigue failure at the tooth root was likely to occur, and the gear fatigue strength was reduced.
  • Comparative steel No. 21 has an Mn content lower than the range of the present invention. Therefore, the hardenability decreased and the effective effect layer depth became shallow, so that the rotational bending fatigue strength and the gear fatigue strength decreased. Since the comparative steel No.
  • Comparative steel No. 22 has a Mn content higher than the range of the present invention, the Ms point of the carburized surface layer portion is lowered and the retained austenite amount is increased. Therefore, the surface hardness was lowered, and the rotational bending fatigue strength and the gear fatigue strength were reduced.
  • Comparative steel No. 23 has an S content higher than the range of the present invention. As a result, the amount of MnS generated as a starting point for fatigue failure increased, and the rotational bending fatigue strength and gear fatigue strength decreased.
  • Comparative steel No. 24 has a Cr content lower than the range of the present invention. Therefore, the core hardness and the resistance to tempering softening decreased, and the rotational bending fatigue strength and the gear fatigue strength decreased. In Comparative Steel Nos.
  • Comparative steel No. 27 has a B content lower than the range of the present invention. Therefore, the hardenability decreased and the effective effect layer depth became shallow, so that the rotational bending fatigue strength and the gear fatigue strength decreased.
  • Comparative steel No. 28 has a B content higher than the range of the present invention. For this reason, the amount of BN produced that caused a decrease in toughness increased, and the rotational bending fatigue strength and gear fatigue strength decreased. In Comparative Steel No.
  • Comparative steel No. 30 has a Sb content lower than the range of the present invention. For this reason, deboronation occurred during carburizing and the surface layer hardness was lowered, so that the rotational bending fatigue strength and the gear fatigue strength were reduced. Comparative steel No. 31 has an N content higher than the range of the present invention.
  • Comparative steel No. 32 has a Ti content higher than the range of the present invention. As a result, fatigue failure due to the TiN starting point was likely to occur, and the rotary bending fatigue strength and gear fatigue strength were reduced. Comparative steel No. 33 is within the composition range of the present invention, but the Sb amount does not satisfy the prescribed formula (Sb ⁇ ⁇ Si / 2 + (Mn + Cr) / 5 ⁇ / 70), so the grain boundary oxide layer Is deep. Therefore, the surface layer hardness was lowered, and the rotational bending fatigue strength and the gear fatigue strength were reduced.

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Abstract

Provided is a case hardened steel which has excellent fatigue characteristics and is obtained at a relatively low production cost. This case hardened steel is configured to have a component composition which contains 0.10-0.30% of C, 0.10-1.20% of Si, 0.30-1.50% of Mn, 0.010-0.030% of S, 0.10-1.00% of Cr, 0.0005-0.0050% of B, 0.005-0.020% of Sb and 0.0150% or less of N, and which additionally contains Al in an amount satisfying 0.010% ≤ Al ≤ 0.120% in cases where B - (10.8/14)N ≥ 0.0003%, and in an amount satisfying 27/14[(N - (14/10.8)B + 0.030] ≤ Al ≤ 0.120% in cases where B - (10.8/14)N < 0.0003%.

Description

肌焼鋼Case-hardened steel
　本発明は、浸炭焼入れして用いられる肌焼鋼、なかでも自動車等の駆動伝達部品に適用できる、耐疲労性および耐衝撃性に優れたボロン含有の肌焼鋼に関するものである。 The present invention relates to a case-hardened steel used by carburizing and quenching, and in particular, a boron-containing case-hardened steel excellent in fatigue resistance and impact resistance, which can be applied to drive transmission parts such as automobiles.
　自動車、建設機械、その他各種の産業機械として用いられる機械部品において、高疲労強度や耐摩耗性が要求される部品には、従来、浸炭、窒化および浸炭窒化などの表面硬化熱処理が施される。これらの用途には、通常、JIS規格でSCr、SCM、SNCMなどの肌焼鋼が用いられ、鍛造や切削等の機械加工により所望の部品形状に成形したのち、上記した表面硬化熱処理を施され、その後、研磨などの仕上げ工程を経て部品へと製造される。近年、自動車、建設機械、その他の産業機械等に使用される部品の製造コストの低減が強く望まれており、鋼材コストの低減、加工工程の合理化および簡略化が進められている。このうち、鋼材コストの低減に関しては、肌焼鋼中のCrやMoの含有量を削減したボロン鋼が種々提案されている。 2. Description of the Related Art Conventionally, surface hardening heat treatments such as carburizing, nitriding, and carbonitriding are performed on parts that require high fatigue strength and wear resistance in machine parts used as automobiles, construction machines, and other various industrial machines. For these applications, case-hardened steel such as SCr, SCM, SNCM, etc. is usually used in JIS standards, and after forming into the desired part shape by machining such as forging or cutting, the above-mentioned surface hardening heat treatment is applied. Then, it is manufactured into parts through a finishing process such as polishing. In recent years, it has been strongly desired to reduce the manufacturing cost of parts used in automobiles, construction machines, other industrial machines, and the like, and the reduction of steel material costs and the rationalization and simplification of processing steps have been promoted. Of these, various boron steels with reduced Cr and Mo contents in case-hardened steel have been proposed for reducing steel material costs.
　例えば、特許文献１には、Tiを添加してＮをTiNの形態で固定し、固溶Ｂを確保しつつ、TiNにより結晶粒の粗大化を抑制可能な肌焼ボロン鋼が開示されている。 For example, Patent Document 1 discloses a case-hardened boron steel that can suppress the coarsening of crystal grains with TiN while adding Ti to fix N in the form of TiN and securing solid solution B. .
　特許文献２には、同じくTi添加型のボロン鋼において、Si、Mn、Crの添加量を調整し、浸炭異常層深さを低減することで、靭性を向上させることが提案されている。 Patent Document 2 proposes to improve toughness by adjusting the addition amount of Si, Mn and Cr in the Ti-added boron steel and reducing the carburizing abnormal layer depth.
　特許文献３には、Alの多量添加によりBNの生成を抑制し、かつ浸炭前の熱処理により得られる微細な炭窒化物により、結晶粒の異常粒成長を防止する肌焼ボロン鋼の製造方法が開示されている。 Patent Document 3 discloses a method for producing a case-hardened boron steel that suppresses the formation of BN by adding a large amount of Al and prevents abnormal grain growth of fine grains by fine carbonitride obtained by heat treatment before carburizing. It is disclosed.
　特許文献４には、Sbの添加により浸炭異常層の発生を抑制し、かつTi-Mo系の炭化物により、結晶粒の粗大化を効果的に抑制する、冷間鍛造性に優れた肌焼鋼が開示されている。
　また、特許文献５には、Sbの添加により脱炭層厚みを抑制し、かつ従来の軟化焼鈍を施した鋼材と同等の冷間加工性を有する機械構造用鋼及びその製造方法が開示されている。 Patent Document 4 discloses a case hardening steel excellent in cold forgeability that suppresses the occurrence of an abnormal carburizing layer by addition of Sb and effectively suppresses coarsening of crystal grains by Ti-Mo carbide. Is disclosed.
Further, Patent Document 5 discloses a steel for machine structural use that suppresses the thickness of the decarburized layer by adding Sb and has a cold workability equivalent to that of a steel material that has been subjected to conventional soft annealing, and a method for manufacturing the same. .
特開昭57－070261号公報JP-A-57-070261 特開昭58－120719号公報JP 58-120719 A 特開2003－342635号公報Japanese Patent Laid-Open No. 2003-342635 特開2012－62536号公報JP 2012-62536 A 特開2004－250767号公報Japanese Patent Laid-Open No. 2004-250767
　しかしながら、上述した特許文献１～４に記載の発明はいずれも、以下に述べる問題があった。
　まず、特許文献１および２に記載の技術では、いずれもＮをTiNの形態で固定し、ＢがＮと結合しないように考慮されている。しかしながら、TiNは比較的大きい角型の介在物として鋼中に存在するため、これが疲労の起点となり、歯車においてはピッチング等の面疲労や歯元の曲げ疲労強度を低下させる。また、角型のTiNは歯車の耐衝撃性を低下させ、歯車に衝撃的な荷重がかかった場合に歯車の折損につながる虞れがある。 However, each of the inventions described in Patent Documents 1 to 4 has the following problems.
First, in the techniques described in Patent Documents 1 and 2, N is fixed in the form of TiN, and it is considered that B does not combine with N. However, since TiN exists in steel as a relatively large square inclusion, this becomes a starting point of fatigue, and in gears, surface fatigue such as pitching and bending fatigue strength at the root are reduced. In addition, the square TiN reduces the impact resistance of the gear and may cause breakage of the gear when an impact load is applied to the gear.
　特許文献３に記載の技術では、微細なAlNやNb(C,N)により、結晶粒の異常成長が抑制されるため、耐衝撃特性を向上することが出来る。しかしながら、浸炭条件によっては、脱ボロンが発生してしまい、表層部が軟化するため、歯面でのピッチングが発生し易くなることが問題になる。 In the technique described in Patent Document 3, since the abnormal growth of crystal grains is suppressed by fine AlN and Nb (C, N), the impact resistance can be improved. However, depending on the carburizing conditions, deboronation occurs and the surface layer portion softens, so that it becomes a problem that pitching on the tooth surface is likely to occur.
　特許文献４に記載の技術では、Sbの添加により、浸炭異常層深さが低減するため、回転曲げ疲労特性を向上することが出来る。しかしながら、浸炭異常層を形成し易いSi、MnおよびCrの含有量が多い場合、上記Sbの効果が得られないことが有り、結果的に疲労強度が低下してしまうという問題がある。 In the technique described in Patent Document 4, the addition of Sb reduces the carburizing abnormal layer depth, and therefore, the rotational bending fatigue characteristics can be improved. However, when the Si, Mn, and Cr contents that are likely to form a carburized abnormal layer are large, the effect of Sb may not be obtained, resulting in a problem that fatigue strength is lowered.
　また、特許文献５に記載の技術では、脱炭抑制効果を有するSbと脱炭を促進するSiとのバランスによっては、表層の炭素の低減を確実に回避することが難しく、所望の特性が得られないという問題がある。 In the technique described in Patent Document 5, depending on the balance between Sb having a decarburization-inhibiting effect and Si that promotes decarburization, it is difficult to reliably avoid the reduction of carbon on the surface layer, and desired characteristics can be obtained. There is a problem that can not be.
　そこで、本発明では、上述した問題を解決し、比較的安価な生産コストで疲労特性に優れた肌焼鋼を提供することを目的とする。 Therefore, an object of the present invention is to solve the above-described problems and to provide a case-hardened steel having excellent fatigue characteristics at a relatively low production cost.
　発明者らは、上述した観点から耐疲労性に優れた肌焼鋼およびその製造方法を開発すべく鋭意研究を重ねた。その結果、以下のことを見出した。
（ａ）AlがＮを固定したときに生成するAlNは、TiがＮを固定して生成する比較的大型なTiN介在物とは異なり、微細な析出物となる。そのために、疲労強度や靱性を低下させる原因とならないばかりか、逆に結晶粒を微細化することによって疲労強度や靱性を向上させる効果を有する。
（ｂ）Tiを添加せず、固溶Ｂの含有量を焼入れ性に効果のある３ppm以上確保するため、鋼中におけるAl－Ｂ－Ｎの化学平衡に基づき、Al含有量を厳密に制御する必要がある。
（ｃ）Ｂはその反応性のゆえ、浸炭時に鋼材表面にて酸化や脱ボロン、窒化等の変化が生じ、表層部の焼入れ性を確保することが難しい。これに対し、Sbを添加することで上記反応を抑制することができる。
（ｄ）Si、MnおよびCrは、焼戻し軟化抵抗の向上に有効であるが、過剰に添加すると、曲げ疲労および疲労亀裂の起点となる粒界酸化を助長する。これに対し、Si、MnおよびCrの含有量に応じてSbを添加することで上記反応を抑制することができる。 Inventors repeated earnest research in order to develop the case hardening steel excellent in fatigue resistance from the viewpoint mentioned above, and its manufacturing method. As a result, the following was found.
(A) AlN produced when Al fixes N is different from a relatively large TiN inclusion produced by Ti fixing N, and becomes a fine precipitate. Therefore, not only does it not cause a decrease in fatigue strength and toughness, but also has the effect of improving the fatigue strength and toughness by refining crystal grains.
(B) Without adding Ti, in order to secure a solid solution B content of 3 ppm or more effective in hardenability, the Al content is strictly controlled based on the chemical equilibrium of Al—BN in steel. There is a need.
(C) Due to the reactivity of B, changes such as oxidation, deboronization, and nitriding occur on the steel surface during carburization, and it is difficult to ensure the hardenability of the surface layer portion. On the other hand, the reaction can be suppressed by adding Sb.
(D) Si, Mn, and Cr are effective in improving the temper softening resistance, but when added in excess, promotes intergranular oxidation that becomes the starting point of bending fatigue and fatigue cracks. On the other hand, the reaction can be suppressed by adding Sb according to the contents of Si, Mn and Cr.
　本発明は上記の知見に立脚するものである。
　すなわち、本発明の要旨構成は、次のとおりである。
１．質量％で、
　Ｃ：0.10～0.30％、
　Si：0.10～1.20％、
　Mn：0.30～1.50％、
　Ｓ：0.010～0.030％、
　Cr：0.10～1.00％、
　Ｂ：0.0005～0.0050％、
　Sb：0.005～0.020％および
　Ｎ： 0.0150％以下
を、下記式を満足する範囲の下で含み、さらに、
　Alを、Ｂ－(10.8/14)Ｎ≧0.0003％の場合に0.010％≦Al≦0.120％およびＢ－(10.8/14)Ｎ＜0.0003％の場合に27／14[(Ｎ－(14／10.8)Ｂ＋0.030]≦Al≦0.120％にて含有し、残部は鉄および不可避不純物からなり、
　前記不可避不純物中のTiが、
　Ti：0.005％以下
であることを特徴とする肌焼鋼。
　　　　　　　　　　　　　記
　Sb≧{Si／２＋(Mn＋Cr)／５}／70
The present invention is based on the above findings.
That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.10 to 0.30%,
Si: 0.10 to 1.20%
Mn: 0.30-1.50%
S: 0.010 to 0.030%,
Cr: 0.10 to 1.00%,
B: 0.0005-0.0050%,
Sb: 0.005 to 0.020% and N: 0.0150% or less are included within a range satisfying the following formula,
When B- (10.8 / 14) N ≧ 0.0003%, 0.010% ≦ Al ≦ 0.120% and B- (10.8 / 14) N <0.0003%, 27/14 [((N− (14 / 10.8 ) B + 0.030] ≦ Al ≦ 0.120%, the balance is iron and inevitable impurities,
Ti in the inevitable impurities,
Ti: Case-hardened steel characterized by being 0.005% or less.
Sb ≧ {Si / 2 + (Mn + Cr) / 5} / 70
２．さらに、質量％で
　Nb：0.050％以下および
　Ｖ：0.200％以下
のいずれか１種または２種を含有する前記１に記載の肌焼鋼。 2. Furthermore, the case hardening steel of said 1 containing the 1 type (s) or 2 types of Nb: 0.050% or less and V: 0.200% or less in the mass%.
　本発明によれば、自動車や産業機械等に使用して好適な疲労強度に優れた肌焼鋼の提供を、量産化の下で実現することができる。 According to the present invention, it is possible to provide a case-hardened steel excellent in fatigue strength suitable for use in automobiles, industrial machines and the like under mass production.
浸炭焼入れ・焼戻し処理条件を示す図である。It is a figure which shows carburizing quenching and tempering process conditions. 小野式回転曲げ疲労試験片の形状を示す図である。It is a figure which shows the shape of an Ono type | formula rotation bending fatigue test piece.
　以下、本発明を具体的に説明する。
　まず、本発明において、鋼の成分組成を上記の範囲に限定した理由について説明する。なお、成分に関する「％」表示は特に断らない限り質量％を意味するものとする。
Ｃ： 0.10～0.30％
　浸炭処理後の焼入れにより該焼入れ材の中心部（以下、単に芯部と示す）の硬度を高めるためには0.10％以上のＣを必要とする。一方、含有量が0.30％を超えると芯部の靭性が低下する。従って、Ｃ量は0.10～0.30％の範囲に限定した。好ましくは0.15～0.25％の範囲である。 Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.10 to 0.30%
In order to increase the hardness of the center portion (hereinafter simply referred to as a core portion) of the quenched material by quenching after carburizing treatment, C of 0.10% or more is required. On the other hand, if the content exceeds 0.30%, the toughness of the core part decreases. Therefore, the C content is limited to the range of 0.10 to 0.30%. Preferably it is 0.15 to 0.25% of range.
Si：0.10～1.20％
　Siは、歯車等が転動中に到達すると推定される、200～300℃の温度域における軟化抵抗を高めるのに有効な元素である。また、浸炭時に粗大な炭化物の生成を抑制する効果も有しており、少なくとも0.10％の添加が不可欠である。一方で、Siはフェライト安定化元素であり、過剰な添加はＡｃ３変態点を上昇させ、通常の焼入れ温度範囲において、炭素含有量の低い芯部でフェライトが出現し易くなり、歯元での曲げ疲労強度が低下するため、上限を1.20％とした。好ましくは0.20～0.60％の範囲である。 Si: 0.10 to 1.20%
Si is an element effective for increasing the softening resistance in a temperature range of 200 to 300 ° C., which is estimated to be reached during rolling of gears and the like. It also has an effect of suppressing the formation of coarse carbides during carburization, and at least 0.10% addition is essential. On the other hand, Si is a ferrite stabilizing element, and excessive addition raises the Ac 3 transformation point, and in the normal quenching temperature range, ferrite tends to appear in the core portion having a low carbon content, and at the tooth base. Since the bending fatigue strength decreases, the upper limit was made 1.20%. Preferably it is 0.20 to 0.60% of range.
Mn：0.30～1.50％
　Mnは、焼入性の向上に有効な元素であり、少なくとも0.30％の添加を必要とする。しかしながら、Mnは、浸炭異常層を形成し易く、また過剰な添加は残留オーステナイト量が過多となって硬さの低下を招くことから、上限を1.50％とした。好ましくは0.50～1.20％の範囲である。 Mn: 0.30 to 1.50%
Mn is an element effective for improving the hardenability and needs to be added at least 0.30%. However, Mn tends to form an abnormal carburization layer, and excessive addition causes an excessive amount of retained austenite and leads to a decrease in hardness, so the upper limit was made 1.50%. Preferably it is 0.50 to 1.20% of range.
Ｓ：0.010～0.030％
　Ｓは、Mnと硫化物を形成し、被削性を向上させる作用を有するため、少なくとも0.010％以上含有させる。一方、過剰な添加は、部品の疲労強度および靭性を低下させるため、上限を0.030％とした。 S: 0.010-0.030%
S has a function of forming sulfides with Mn and improving machinability, so it is contained at least 0.010% or more. On the other hand, excessive addition reduces the fatigue strength and toughness of the parts, so the upper limit was made 0.030%.
Cr：0.10～1.00％
　Crは、焼入性のみならず焼戻し軟化抵抗の向上にも有効な元素であり、含有量が0.10％に満たないとその添加効果に乏しい。一方、1.00％を超えると、浸炭異常層を形成し易くなる。さらに、焼入れ性が高くなりすぎて、歯車内部の靭性が劣化し、曲げ疲労強度が低下することになる。従って、Cr量は0.10～1.00％の範囲に限定した。好ましくは0.10～0.60％の範囲である。 Cr: 0.10 to 1.00%
Cr is an element effective for improving not only hardenability but also temper softening resistance, and if the content is less than 0.10%, its addition effect is poor. On the other hand, when it exceeds 1.00%, it becomes easy to form a carburized abnormal layer. Further, the hardenability becomes too high, the toughness inside the gear is deteriorated, and the bending fatigue strength is lowered. Therefore, the Cr content is limited to the range of 0.10 to 1.00%. Preferably it is 0.10 to 0.60% of range.
Ｂ： 0.0005～0.0050％
　Ｂは、微量の添加により焼入れ性を確保するのに有効な元素であり、少なくとも0.0005％の添加を必要とする。一方、0.0050％を超えると、ＢＮの量が増えてしまい、部品の疲労強度および靭性を低下させるため、Ｂ量は0.0005～0.0050％の範囲に限定した。好ましくは0.0010～0.0040％の範囲である。 B: 0.0005-0.0050%
B is an element effective for ensuring hardenability by adding a small amount, and needs to be added at least 0.0005%. On the other hand, if it exceeds 0.0050%, the amount of BN increases and the fatigue strength and toughness of the parts are reduced. Therefore, the amount of B is limited to the range of 0.0005 to 0.0050%. Preferably it is 0.0010 to 0.0040% of range.
Sb：0.005～0.020％
　Sbは、粒界への偏析傾向が強いため、浸炭処理中の脱ボロン、窒化(BN形成)等の表層反応を抑制し、焼入れ性を確保するために重要な元素である。その効果を得るには、少なくとも0.005％の添加が不可欠である。しかしながら、過剰な添加はコスト増につながるだけでなく、靭性を低下させるため、上限を0.020％とした。好ましくは0.005～0.015％の範囲である。 Sb: 0.005-0.020%
Since Sb has a strong tendency to segregate at grain boundaries, Sb is an important element for suppressing surface reaction such as deboronization and nitriding (BN formation) during carburizing treatment and ensuring hardenability. In order to obtain the effect, the addition of at least 0.005% is essential. However, excessive addition not only leads to an increase in cost but also reduces toughness, so the upper limit was made 0.020%. Preferably it is 0.005 to 0.015% of range.
　さらに、Sbについては、上記したSi、MnおよびCrの含有量に関する、次式
　Sb≧{Si／２＋(Mn＋Cr)／５}／70
の関係を満足させることが重要である。すなわち、上式は、粒界酸化層深さに影響を与える因子を示していて、SbがSi、MnおよびCr含有量に関する規定値を満たさない場合、粒界酸化の抑制効果に乏しく、疲労特性の低下を招くことになる。
　ここで、粒界酸化とは、浸炭処理等の熱処理において鋼材の表層部の結晶粒界が内部酸化する現象であり、鋼中に選択酸化され易いSiやCr等が存在していると、その生成を助長する。粒界酸化部では上記の元素が酸化により消費されてしまうため、周辺部での焼入れ性低下に伴い硬度が低下し、そこを起点とした疲労破壊が起こりやすくなる。本発明では、粒界酸化の抑制作用を有するSbの添加量の下限をSi、Mn、Crの含有量に応じて上記式の右辺で示すように特定することによって、表層での焼入れ性を確保でき、疲労強度の低下を抑制できる。 Further, for Sb, the following formula relating to the contents of Si, Mn and Cr described above: Sb ≧ {Si / 2 + (Mn + Cr) / 5} / 70
It is important to satisfy this relationship. That is, the above equation shows the factors that affect the grain boundary oxide layer depth, and when Sb does not meet the specified values for Si, Mn, and Cr contents, the effect of suppressing grain boundary oxidation is poor, and the fatigue characteristics Will be reduced.
Here, grain boundary oxidation is a phenomenon in which the grain boundary of the surface layer portion of the steel material is internally oxidized in heat treatment such as carburizing treatment, and if there is Si or Cr which is easily oxidized in the steel, Contributes to generation. Since the above elements are consumed by oxidation in the grain boundary oxidation portion, the hardness is lowered with a decrease in the hardenability in the peripheral portion, and fatigue failure starting from the hardness tends to occur. In the present invention, by specifying the lower limit of the amount of Sb having the effect of suppressing grain boundary oxidation as indicated by the right side of the above formula according to the content of Si, Mn, Cr, ensuring hardenability in the surface layer It is possible to suppress a decrease in fatigue strength.
Ｎ： 0.0150％以下
　Ｎは、Alと結合してAlNを形成し、オーステナイト結晶粒の微細化に寄与する元素である。そのためには、0.0030％以上で添加することが好ましい。しかし、過剰に添加すると固溶Ｂの確保が困難になるだけでなく、凝固時の鋼塊に気泡が発生したり、鍛造性の劣化を招くため、上限を0.0150％とする。 N: 0.0150% or less N is an element that combines with Al to form AlN and contributes to the refinement of austenite crystal grains. Therefore, it is preferable to add at 0.0030% or more. However, when added in excess, not only is it difficult to secure the solid solution B, but also bubbles are generated in the steel ingot during solidification and deterioration of forgeability is caused, so the upper limit is made 0.0150%.
　Alの含有量は、Ｂ量に応じて、次のとおりに規定する。
Ｂ－(10.8/14)Ｎ≧0.0003％の場合：0.010％≦Al≦0.120％　Alは、脱酸剤として必要な元素であると同時に、本発明においては固溶Ｂを確保するためにも必要な元素である。ここで、「Ｂ－(10.8/14)Ｎ」は、含有Ｂのうち化学量論的にＮと結合するＢ量を差し引いた残部のＢ量（以下、固溶Ｂ量ともいう）を表している。
　この固溶Ｂ量が0.0003％以上であれば、焼入れ性向上に必要な固溶Ｂの確保が可能となる。この場合において、Al含有量が0.010％未満であると、脱酸が不十分になり、酸化物系介在物による疲労強度の低下をまねくことになる。一方、0.120％を超えてAlを添加すると、連続鋳造時のノズル詰まりの発生やアルミナクラスター介在物の発現により、靱性の低下を招く。よって、固溶Ｂ量が0.0003％以上のとき、Al含有量は0.010％以上0.120％以下の範囲とする。 The content of Al is specified as follows according to the amount of B.
When B- (10.8 / 14) N ≧ 0.0003%: 0.010% ≦ Al ≦ 0.120% Al is an element necessary as a deoxidizing agent, and at the same time, in the present invention, it is necessary to secure solid solution B. Element. Here, “B- (10.8 / 14) N” represents the remaining B amount after subtracting the stoichiometric amount of B that binds to N from the contained B (hereinafter also referred to as solid solution B amount). Yes.
If this solid solution B amount is 0.0003% or more, it becomes possible to secure the solid solution B necessary for improving the hardenability. In this case, if the Al content is less than 0.010%, deoxidation becomes insufficient, and the fatigue strength is reduced due to oxide inclusions. On the other hand, when Al is added exceeding 0.120%, the toughness is reduced due to the occurrence of nozzle clogging during continuous casting and the appearance of alumina cluster inclusions. Therefore, when the solid solution B content is 0.0003% or more, the Al content is set to a range of 0.010% or more and 0.120% or less.
Ｂ－(10.8/14)Ｎ＜0.0003％の場合：27／14[(Ｎ－(14／10.8)Ｂ＋0.030]≦Al≦0.120％　上記に対し、固溶Ｂ量が0.0003％未満の場合は、他にＮと結合し易い合金元素がない限り、Ｎは全量がＢと結合するため、固溶Ｂを確保することが難しくなる。
　この場合は、Ｎと比較的結合し易いAlの量を増やし、焼入れ性向上に寄与する固溶Ｂ量を確保する必要がある。そのために、Al含有量を27／14[(Ｎ－(14／10.8)Ｂ＋0.030]％以上として0.0003％以上の固溶Ｂ量を確保する。なお、Alの上限は、上記と同様に0.120％とする。 When B- (10.8 / 14) N <0.0003%: 27/14 [(N- (14 / 10.8) B + 0.030] ≦ Al ≦ 0.120% On the other hand, when the amount of dissolved B is less than 0.0003% As long as there is no other alloying element that can be easily combined with N, the entire amount of N is combined with B, so that it is difficult to ensure solid solution B.
In this case, it is necessary to increase the amount of Al that is relatively easy to bond with N and to secure a solid solution B amount that contributes to improving the hardenability. Therefore, the Al content is 27/14 [(N- (14 / 10.8) B + 0.030]% or more, and a solid solution B amount of 0.0003% or more is ensured. %.
　上記した成分の残部は、鉄および不可避不純物であるが、この不純物のうちTiは、以下に示す上限に従って抑制する必要がある。 The balance of the above-described components is iron and inevitable impurities, and Ti among these impurities must be suppressed according to the upper limit shown below.
Ti：0.005％以下
　TiはＮとの結合力が強く、TiNを形成する。しかし、TiNは比較的大きい角型の介在物として鋼中に存在するため、これが疲労の起点となり、歯車においてはピッチング等の面疲労や歯元の曲げ疲労強度を低下させる。従って、本発明においてTiは不純物であり、できるだけ少ない方がよい。具体的には、0.005％を超えると、上記弊害が現れるため、Ti量は0.005％以下に限定する。 Ti: 0.005% or less Ti has a strong bonding force with N and forms TiN. However, since TiN exists in steel as a relatively large square inclusion, this becomes a starting point of fatigue, and in gears, surface fatigue such as pitching and bending fatigue strength at the root are reduced. Accordingly, in the present invention, Ti is an impurity, and it is preferable that Ti be as small as possible. Specifically, if it exceeds 0.005%, the above-mentioned adverse effects appear, so the Ti amount is limited to 0.005% or less.
　この他、不可避不純物としては、ＰおよびＯが挙げられる。
　すなわち、Ｐは、粒界に偏析し、浸炭層及び内部の靭性を低下させる原因となるため、低いほど望ましい。具体的には、0.020％を超えると、上記弊害が現れるため、Ｐ量は0.020％以下とすることが好ましい。 In addition, P and O are mentioned as inevitable impurities.
That is, P is segregated at the grain boundary and causes the carburized layer and the internal toughness to be lowered. Specifically, when the content exceeds 0.020%, the above-described adverse effects appear, so the P content is preferably set to 0.020% or less.
　また、Ｏは、鋼中において酸化物系介在物として存在し、疲労強度を損なう元素である。TiN介在物と同様に、疲労強度及び靭性を低下させる原因となるため、低いほど望ましい。具体的には0.0020％を超えると、上記弊害が現れるため、Ｏ量は0.0020％以下とすることが好ましい。 O is an element that exists as an oxide inclusion in steel and impairs fatigue strength. Like TiN inclusions, the lower the content, the lower the fatigue strength and toughness. Specifically, if it exceeds 0.0020%, the above-described adverse effects appear, so the O content is preferably 0.0020% or less.
　以上が本発明の基本成分組成であるが、さらに特性を向上させる場合に、NbおよびＶのいずれか１種または２種を含有してもよい。
Nb：0.050％以下
　Nbは、結晶粒を微細化し、粒界を強化して疲労強度向上に寄与するため添加してもよく、添加する場合は、少なくとも0.010％以上で含有させることが好ましい。一方、その効果は0.050％で飽和し、かつ多量の添加はコスト増になるため、上限を0.050％とすることが好ましい。 The above is the basic component composition of the present invention, but when further improving the characteristics, either one or two of Nb and V may be contained.
Nb: 0.050% or less Nb may be added to refine crystal grains and strengthen grain boundaries to contribute to improving fatigue strength. When Nb is added, it is preferably contained at least 0.010% or more. On the other hand, the effect is saturated at 0.050%, and addition of a large amount increases the cost, so the upper limit is preferably made 0.050%.
Ｖ：0.200％以下
　Ｖは、焼入れ性を向上させると共にSiやCrと同じく焼戻し軟化抵抗を高める元素であり、炭窒化物を形成して結晶粒の粗大化を抑制する効果も有する。このような効果を発揮させるためには、0.030％以上で添加することが好ましい。また、その効果は0.200％で飽和し、かつ多量の添加はコスト増になるため、添加する場合は、0.200％以下とすることが好ましい。
　なお、被削性を向上させるためには、必要に応じて、Pb、Se、Ca等の快削元素を含有させてもよい。 V: 0.200% or less V is an element that improves hardenability and increases temper softening resistance like Si and Cr, and also has an effect of forming carbonitrides and suppressing coarsening of crystal grains. In order to exert such an effect, it is preferable to add at 0.030% or more. Further, the effect is saturated at 0.200%, and the addition of a large amount increases the cost. Therefore, when added, the content is preferably 0.200% or less.
In order to improve machinability, a free cutting element such as Pb, Se, or Ca may be included as necessary.
　本発明に係る肌焼鋼から機械構造用部品を作製する際の製造条件については、特に制限は無いが、好適な製造条件は次の通りである。
　前記した成分組成からなる鋼素材を溶解鋳造してビレットとし、このビレットを熱間圧延後、歯車とするための予備成形を行う。次に、機械加工、あるいは鍛造後に機械加工を行い歯車形状とした後、浸炭焼入れ処理を施し、必要に応じて更に歯面に研磨加工を施して最終製品とする。更には、ショットピーニング等を付加しても良い。浸炭焼入れ処理は、浸炭温度900～1050℃、焼入れ温度800～900℃とし、焼戻しは120～250℃の範囲とすることが好ましい。 Although there is no restriction | limiting in particular about the manufacturing conditions at the time of producing the machine structural component from the case hardening steel based on this invention, The suitable manufacturing conditions are as follows.
A steel material having the above-described component composition is melt-cast to form a billet, and this billet is hot-rolled and then preformed to form a gear. Next, it is machined or machined after forging to form a gear shape, and then carburized and quenched, and if necessary, the tooth surface is further polished to obtain a final product. Furthermore, shot peening or the like may be added. The carburizing and quenching treatment is preferably performed at a carburizing temperature of 900 to 1050 ° C., a quenching temperature of 800 to 900 ° C., and tempering within a range of 120 to 250 ° C.
　表１に示す化学組成の鋼を溶製し鋳造によってビレットとし、このビレットを熱間圧延により20mmφ、32mmφおよび70mmφの棒鋼に加工し、得られた丸棒鋼に対し、925℃で焼準処理を実施した。表１中に示すNo.１～15は本発明の成分組成に従う発明鋼であり、No.16～33は本発明の規制値から外れた含有量の成分を含む比較鋼であり、No.34はJIS　SCr420規格材である。　焼準処理後の丸棒より、小野式回転曲げ疲労試験片および歯車疲労試験片を採取した。表1の成分組成を有する各試験片に対して、図１に示す条件に従って、浸炭焼入れ・焼戻しを施した後、粒界酸化層深さ、有効硬化層深さ、表面硬度、内部硬度の各調査及び回転曲げ疲労試験、歯車疲労試験を実施した。以下に、それぞれの調査内容について詳細に説明する。 Steel with the chemical composition shown in Table 1 is melted and cast into billets. These billets are processed into 20 mmφ, 32 mmφ, and 70 mmφ steel bars by hot rolling, and the resulting round steel bars are subjected to normalization at 925 ° C. Carried out. Nos. 1 to 15 shown in Table 1 are invention steels according to the composition of the present invention, and Nos. 16 to 33 are comparative steels containing components whose contents deviate from the regulation values of the present invention. Is JIS SCr420 standard material. From the round bars after the normalization treatment, Ono type rotating bending fatigue test pieces and gear fatigue test pieces were collected. Each test piece having the composition shown in Table 1 is subjected to carburizing and tempering according to the conditions shown in FIG. 1, and then each of the grain boundary oxide layer depth, effective hardened layer depth, surface hardness, and internal hardness is measured. Investigation, rotation bending fatigue test, and gear fatigue test were conducted. The details of each survey are described below.
［粒界酸化層深さ、有効硬化層深さ、表面硬度、内部硬度］
　発明鋼、比較鋼及びSCr420の20mmφ丸棒に、浸炭焼入れ・焼戻し処理を施した後に切断し、この切断面において最大となる粒界酸化層深さを、エッチングすることなく光学顕微鏡で400倍の倍率にて測定した。
　また、同じ断面の硬度分布を測定し、ビッカース硬さで550HVとなる表面からの深さを有効硬化層深さとした。表面硬度は、丸棒表面のビッカース硬さ(HV10kgf)10点の平均値とした。さらに、表層より５mm深さ位置のビッカース硬さ(HV10kgf)５点の平均値を内部硬度と規定した。 [Grain boundary oxide layer depth, effective hardened layer depth, surface hardness, internal hardness]
Invented steel, comparative steel and SCr420 20mmφ round bar were cut after carburizing quenching and tempering treatment, and the maximum grain boundary oxide layer depth in this cut surface was 400 times by optical microscope without etching. Measured with magnification.
The hardness distribution of the same cross section was measured, and the depth from the surface where the Vickers hardness was 550 HV was defined as the effective hardened layer depth. The surface hardness was an average value of 10 points of Vickers hardness (HV 10 kgf) on the surface of the round bar. Furthermore, the average value of five points of Vickers hardness (HV 10 kgf) at a depth of 5 mm from the surface layer was defined as the internal hardness.
［回転曲げ疲労特性］
　直径32mmの丸棒鋼から、平行部が圧延方向と一致するように、図２に示す寸法および形状の平行部直径８mmの試験片を採取し、平行部にこれと直角方向の深さ２mmの切欠き（切欠き係数：1.56）を全周に付与した回転曲げ疲労試験片を作製した。得られた試験片に対して、浸炭焼入れ・焼戻し処理を行った後、小野式回転曲げ疲労試験機を用い、回転数：3000rpmで回転曲げ疲労試験を実施し、10７回を疲労限度として、回転曲げ疲労強度を測定した。 [Rotating bending fatigue characteristics]
From a round steel bar with a diameter of 32 mm, a test piece with a parallel part diameter of 8 mm having the dimensions and shape shown in Fig. 2 is taken so that the parallel part coincides with the rolling direction. A rotating bending fatigue test piece having notches (notch coefficient: 1.56) all around was prepared. The obtained test pieces, after carburizing quenching and tempering process, using a fatigue tester Ono-type rotating bending, rotation speed: conducted rotary bending fatigue test at 3000 rpm, 10 7 times as fatigue limit, The rotational bending fatigue strength was measured.
［歯車疲労特性］
　直径70mmの丸棒を熱間鍛造後に機械加工して、モジュール2.5、ピッチ直径80mmのハスバ歯車を作製した。得られた試験片に対して、動力循環式歯車疲労試験機を使用して、80℃のトランスアクスルオイルを潤滑に用い、所定のトルクをかけて回転数：3000rpmにて試験を実施し、10７回を疲労限度として、歯車疲労強度を測定した。 [Gear fatigue characteristics]
A round bar with a diameter of 70 mm was machined after hot forging to produce a helical gear with a module 2.5 and a pitch diameter of 80 mm. Using the power cycle type gear fatigue tester, the test piece obtained was tested at 80 ° C. transaxle oil for lubrication, applying a predetermined torque and rotating at 3000 rpm. The gear fatigue strength was measured with 7 times as the fatigue limit.
［調査結果］
　上記した調査項目毎の調査結果を、表２に示す。本発明鋼(No.１～15)は、回転曲げ/歯車疲労特性共にSCr420(No.34)と同等以上の特性が得られており、比較鋼(No．16～33)より優れていることがわかる。 [Investigation result]
Table 2 shows the survey results for each of the survey items described above. The steels of the present invention (Nos. 1 to 15) have the same or better properties than SCr420 (No. 34) in both rotational bending and gear fatigue characteristics, and are superior to the comparative steels (No. 16 to 33). I understand.
　すなわち、比較鋼No.16はＣ含有量が本発明範囲より低いために、内部硬度が低くなりすぎ、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.17は、Ｃ含有量が本発明範囲より高いために、芯部の靭性が低下し、回転曲げ疲労強度および歯車疲労強度が低下した。
　比較鋼No.18は、Si含有量が本発明の範囲よりも低いために、耐焼戻し軟化抵抗が低下し、歯車疲労強度が低下した。
　比較鋼No.19は、Si含有量が本発明の範囲よりも低くかつCr含有量が本発明の範囲より高い。そのため、浸炭表層部のMs点が低下し、残留オーステナイト量が増加する。よって、表層硬度が低くなり、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.20は、Si含有量が本発明の範囲よりも高い。そのため、内部にフェライトが発生し、歯元での曲げ疲労破壊が起こりやすくなり、歯車疲労強度が低下した。
　比較鋼No.21は、Mn含有量が本発明範囲より低い。そのため、焼入れ性が低下し、有効効果層深さが浅くなったため、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.22は、Mn含有量が本発明の範囲より高いために、浸炭表層部のMs点が低下し、残留オーステナイト量が増加する。よって、表面硬度が低くなり、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.23は、Ｓ含有量が本発明範囲より高い。そのため、疲労破壊の起点となるMnSの生成量が多くなり、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.24は、Cr含有量が本発明の範囲より低い。そのため、芯部硬度及び耐焼戻し軟化抵抗が低下し、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.25および26は、Cr含有量が本発明の範囲より高いために、浸炭表層部のMs点が低下し、残留オーステナイト量が増加する。よって、表層硬度が低くなり、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.27は、Ｂ含有量が本発明の範囲より低い。そのため、焼入れ性が低下し、有効効果層深さが浅くなったため、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.28は、Ｂ含有量が本発明の範囲より高い。そのため、靭性の低下を招くＢＮの生成量が多くなり、回転曲げ疲労強度および歯車疲労強度が低下した。
　比較鋼No.29は、Al含有量が本発明で規定した式(27/14[(N-(14 /10.8)B＋0.030]≦Al≦0.120％)から算出される下限値より低い。そのため、焼入れ性向上に寄与する固溶Ｂ量が確保できず、有効効果層深さが浅く、内部硬度も低くなったため、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.30は、Sb含有量が本発明範囲より低い。そのため、浸炭時に脱ボロンが生じてしまい、表層硬度が低くなったため、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.31は、Ｎ含有量が本発明の範囲より高い。その結果、焼入れ性向上に寄与する固溶Ｂ量が確保できず、有効効果層深さが浅く、内部硬度も低くなったため、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.32は、Ti含有量が本発明の範囲より高い。そのため、TiN起点による疲労破壊が起こりやすくなり、回転曲げ疲労強度と歯車疲労強度が低下した。
　比較鋼No.33は、本発明成分範囲内であるが、Sb量が規定式(Sb≧{Si/2+(Mn+Cr)/5}/70)を満たしていないため、粒界酸化層が深い。よって、表層硬度が低くなり、回転曲げ疲労強度と歯車疲労強度が低下した。 That is, since the comparative steel No. 16 had a C content lower than the range of the present invention, the internal hardness was too low, and the rotational bending fatigue strength and the gear fatigue strength were reduced.
Since the comparative steel No. 17 had a C content higher than the range of the present invention, the toughness of the core portion was reduced, and the rotary bending fatigue strength and the gear fatigue strength were reduced.
In Comparative Steel No. 18, since the Si content was lower than the range of the present invention, the resistance to tempering softening decreased and the gear fatigue strength decreased.
Comparative Steel No. 19 has a Si content lower than the range of the present invention and a Cr content higher than the range of the present invention. For this reason, the Ms point of the carburized surface layer portion decreases and the amount of retained austenite increases. Therefore, the surface layer hardness was lowered, and the rotational bending fatigue strength and the gear fatigue strength were reduced.
Comparative steel No. 20 has a Si content higher than the range of the present invention. Therefore, ferrite was generated inside, bending fatigue failure at the tooth root was likely to occur, and the gear fatigue strength was reduced.
Comparative steel No. 21 has an Mn content lower than the range of the present invention. Therefore, the hardenability decreased and the effective effect layer depth became shallow, so that the rotational bending fatigue strength and the gear fatigue strength decreased.
Since the comparative steel No. 22 has a Mn content higher than the range of the present invention, the Ms point of the carburized surface layer portion is lowered and the retained austenite amount is increased. Therefore, the surface hardness was lowered, and the rotational bending fatigue strength and the gear fatigue strength were reduced.
Comparative steel No. 23 has an S content higher than the range of the present invention. As a result, the amount of MnS generated as a starting point for fatigue failure increased, and the rotational bending fatigue strength and gear fatigue strength decreased.
Comparative steel No. 24 has a Cr content lower than the range of the present invention. Therefore, the core hardness and the resistance to tempering softening decreased, and the rotational bending fatigue strength and the gear fatigue strength decreased.
In Comparative Steel Nos. 25 and 26, since the Cr content is higher than the range of the present invention, the Ms point of the carburized surface layer portion decreases and the retained austenite amount increases. Therefore, the surface layer hardness was lowered, and the rotational bending fatigue strength and the gear fatigue strength were reduced.
Comparative steel No. 27 has a B content lower than the range of the present invention. Therefore, the hardenability decreased and the effective effect layer depth became shallow, so that the rotational bending fatigue strength and the gear fatigue strength decreased.
Comparative steel No. 28 has a B content higher than the range of the present invention. For this reason, the amount of BN produced that caused a decrease in toughness increased, and the rotational bending fatigue strength and gear fatigue strength decreased.
In Comparative Steel No. 29, the Al content is lower than the lower limit value calculated from the formula (27/14 [(N− (14 / 10.8) B + 0.030] ≦ Al ≦ 0.120%) defined in the present invention. Rotational bending fatigue strength and gear fatigue strength decreased because the amount of solute B that contributed to improving hardenability could not be secured, the effective effect layer depth was shallow, and the internal hardness was low.
Comparative steel No. 30 has a Sb content lower than the range of the present invention. For this reason, deboronation occurred during carburizing and the surface layer hardness was lowered, so that the rotational bending fatigue strength and the gear fatigue strength were reduced.
Comparative steel No. 31 has an N content higher than the range of the present invention. As a result, the amount of solute B contributing to the improvement of hardenability could not be secured, the effective effect layer depth was shallow, and the internal hardness was low, so that the rotational bending fatigue strength and the gear fatigue strength were reduced.
Comparative steel No. 32 has a Ti content higher than the range of the present invention. As a result, fatigue failure due to the TiN starting point was likely to occur, and the rotary bending fatigue strength and gear fatigue strength were reduced.
Comparative steel No. 33 is within the composition range of the present invention, but the Sb amount does not satisfy the prescribed formula (Sb ≧ {Si / 2 + (Mn + Cr) / 5} / 70), so the grain boundary oxide layer Is deep. Therefore, the surface layer hardness was lowered, and the rotational bending fatigue strength and the gear fatigue strength were reduced.
Figure JPOXMLDOC01-appb-T000001
  
Figure JPOXMLDOC01-appb-T000001
  
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002

Claims (2)

  1. 　質量％で、
    　Ｃ：0.10～0.30％、
    　Si：0.10～1.20％、
    　Mn：0.30～1.50％、
    　Ｓ：0.010～0.030％、
    　Cr：0.10～1.00％、
    　Ｂ：0.0005～0.0050％、
    　Sb：0.005～0.020％および
    　Ｎ： 0.0150％以下
    を、下記式を満足する範囲の下で含み、さらに、
    　Alを、Ｂ－(10.8/14)Ｎ≧0.0003％の場合に0.010％≦Al≦0.120％およびＢ－(10.8/14)Ｎ＜0.0003％の場合に27／14[(Ｎ－(14／10.8)Ｂ＋0.030]≦Al≦0.120％にて含有し、残部は鉄および不可避不純物からなり、
    　前記不可避不純物中のTiが、
    　Ti：0.005％以下
    であることを特徴とする肌焼鋼。
    　　　　　　　　　　　　　記
    　Sb≧{Si／２＋(Mn＋Cr)／５}／70     % By mass
    C: 0.10 to 0.30%,
    Si: 0.10 to 1.20%
    Mn: 0.30-1.50%
    S: 0.010 to 0.030%,
    Cr: 0.10 to 1.00%,
    B: 0.0005-0.0050%,
    Sb: 0.005 to 0.020% and N: 0.0150% or less are included within a range satisfying the following formula,
    When B- (10.8 / 14) N ≧ 0.0003%, 0.010% ≦ Al ≦ 0.120% and B- (10.8 / 14) N <0.0003%, 27/14 [((N− (14 / 10.8 ) B + 0.030] ≦ Al ≦ 0.120%, the balance is iron and inevitable impurities,
    Ti in the inevitable impurities,
    Ti: Case-hardened steel characterized by being 0.005% or less.
    Sb ≧ {Si / 2 + (Mn + Cr) / 5} / 70
  2. 　さらに、質量％で
    　Nb：0.050％以下および
    　Ｖ：0.200％以下
    のいずれか１種または２種を含有する請求項１に記載の肌焼鋼。     Furthermore, the case hardening steel of Claim 1 which contains any 1 type or 2 types of Nb: 0.050% or less and V: 0.200% or less in the mass%.
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